It has been discovered that magnesium diboride is a superconductor with a transition temperature of approximately 40 K. Magnesium diboride can be made by the reaction of elemental magnesium and boron. The result of this process is a fine powder which is commercially available. Experiments on small crystals of this material have demonstrated high current-carrying capabilities at high magnetic fields, properties which could make MgB2 very useful in applications such as magnetic resonance imaging (MRI) where large powerful magnets are required. Magnesium diboride, however is an intractable material with respect to the usual drawing processes for forming the continuous wires required for such applications.
Magnesium diboride wires have been formed by a “powder-in-tube” process in which a tube of cladding material is filled with the fine powder and the composite tube is then drawn to smaller diameter. (S. Jin et al, high Critical Currents in Iron-clad Superconducting MgB2 Wires, Nature, Vol. 410, 63(2001)). This process is expensive and may not lead to optimum properties in the fabricated wire.
Another approach to forming MgB2 wires has been to convert boron filaments by reaction with magnesium vapor. Boron filaments are formed in a continuous chemical vapor deposition (CVD) process; 100 micron diameter boron filaments on a 12 micron tungsten substrate are commercially available in lengths exceeding several kilometers. Segments of these filaments were reacted with magnesium vapor in sealed tantalum tubes. (Canfield et al, Superconductivity in Dense MgB2 Wires, Phys. Rev. Lett., Vol. 86, 2424 (2001)). The filament segments retained the shape of wires after conversion to MgB2, and exhibited good superconducting properties. However, the resulting wires were fragile and difficult to handle.
One objective of the invention disclosed below is to form a boron substrate which can be converted to magnesium diboride in continuous wire form while still retaining both good superconducting properties and good mechanical properties such as handleability.
Another aspect of the superconducting behavior of MgB2 is the effect of impurities. Impurity sites can enhance the current-carrying capability of a superconductor by “pinning” magnetic vortices; the restrained vortices allow the sample to retain a zero electrical resistance. (Canfield and Bud'ko, Physics World, 29, Jan. 2001.) Impurities which have been found useful for enhancing the properties of MgB2 include magnesium oxide, carbon, silicon carbide and titanium diboride.
Another objective of this invention is to provide a continuous boron substrate doped in a controlled manner by chemical vapor deposition with atomic species which will, upon conversion of the boron to MgB2, form “pinning” sites which will enhance the current-carrying capability of the resulting superconductor.